Seasonal water storage modulating seismicity on California faults

In California the accumulation of winter snowpack in the Sierra Nevada, surface water in lakes and reservoirs, and groundwater in sedimentary basins follow the annual cycle of wet winters and dry summers. The surface loads resulting from the seasonal changes in water storage produce elastic deformation of the Earth’s crust. Micro-earthquakes in California appear to follow a subtle annual cycle, possibly in response to the water load. Previous studies posit that temperature, atmospheric pressure, or hydrologic changes may strain the lithosphere and promote additional earthquakes above background levels. Here we use GPS vertical time series (2006 – 2015) to constrain models of monthly hydrospheric loading and compute annual peak-to-peak stresses on faults throughout northern California, which can exceed 1kPa. Depending on fault geometry the addition or removal of water increases the Coulomb failure stress. The largest stress amplitudes are occurring on dipping reverse faults in the Coast Ranges and along the eastern Sierra Nevada range front. We analyze M≥2.0 earthquakes with known focal mechanisms in northern and central California to resolve fault normal and shear stresses for the focal geometry. Our results reveal more earthquakes occurring during slip-encouraging stress conditions and suggest that earthquake populations are modulated at periods of natural loading cycles, which promote failure by subtle stress changes. The most notable shear-stress change occurs on more shallowly dipping structures. However, vertically dipping strike-slip faults are common throughout California and experience smaller amplitude stress change but still exhibit positive correlation with seasonal loading cycles. Our seismicity analysis suggests the annual hydrologic cycle is a viable mechanism to promote earthquakes and provides new insight to fault mechanical properties.